The invention generally relates to position sensors, and more particularly, to linear position sensors for use on power cylinders.
Equipment implementing hydraulic cylinders for mechanical movement, such as excavators and other heavy construction equipment, depend upon operators to manually control the moveable elements of the equipment. The operator must manually move control levers to open and close hydraulic valves that direct pressurized fluid to hydraulic cylinders. For example, when the operator lifts a lift arm, the operator actually moves a lever associated with the lift arm causing a valve to release pressurized fluid to the lift arm cylinder. The use of levers to control hydraulic equipment depends upon manual dexterity and requires great skill. Improperly operated equipment poses a safety hazard, and operators have been known to damage overhead utility wires, underground wiring, water mains, and underground gas lines through faulty operation of excavators, bucket loaders or like equipment.
In addition to the safety hazards caused by improperly operated equipment, the machine""s operating efficiency is also a function of the operator""s skill. An inexperienced or unskilled operator typically fails to achieve the optimum performance levels of the equipment. For instance, the operator may not consistently apply the force necessary for peak performance due to a concern over striking a hazard. Efficiency is also compromised when the operator fails to drive a cylinder smoothly. The operator alternately overdrives or underdrives the cylinder, resulting in abrupt starts and stops of the moveable element and thereby derating system performance. As a result, the skill level necessary to properly and safely operate heavy equipment is typically imparted through long and costly training courses and apprenticeships.
There have been various attempts at implementing an automated control system for use on heavy equipment. One such system is disclosed in U.S. Pat. No. 4,288,196. The system described therein provides for a computer programmable system for setting the lowermost point of a backhoe bucket. In U.S. Pat. No. 4,945,221, a control system for an excavator is disclosed. The system attempts to control the position of the bucket cutting edge to a desired depth. Another position locating system for heavy equipment is disclosed in U.S. Pat. No. 5,404,661.
These systems and others like them share a common feature in that they implement a position sensor. Typically, these sensors are rotary potentiometers as, for instance, suggested in Murakmi, Kato and Ots, Precision Angle Sensor Unit for Construction Machinery, SAE Technical Paper Series 972782, 1997. This sensor relies upon a potentiometer which changes a voltage or current in relation to the position of a bucket or boom. Other types of sensors rely upon optical, conductive plastic, or metal-in-glass technologies.
It is a disadvantage of these sensors that they mount to the outside of the machinery, thereby exposing them to the environment. In the case of heavy equipment, this environment includes severe temperatures, excessive moisture, and air-borne particulate matter which may infect the sensor. In the case of optical, conductive plastic and metal-in-glass technologies, the sensors would rapidly degrade if used on construction equipment. Furthermore, some of these sensors use contacting components that are susceptible to wear, vibration and temperature. As a result, no sensor mountable to the outside of heavy equipment or relying upon contacting elements has gained widespread use in the industry.
There have been attempts to overcome the limitations of noncontacting sensors by using electromagnetic energy. For example, the system disclosed in U.S. Pat. No. 4,945,221 discloses using lasers for sensing problems. Others suggest using RF energy or the like to provide a feedback signal. These systems, however, have not replaced the less expensive potentiometers due to their complexity of use and their expense.
As the demands placed upon actuated machinery increases, so does the demand for a low cost, long-life sensor operable in a harsh environment. Despite the development of highly sophisticated control systems, computer processors and application specific software, the implementation of this technology in electrohydraulic equipment has been curtailed by the failure to provide a long-life, cost-effective precision sensor operable in harsh environments.
A sensor according to the principles of the invention provides a precision signal utilizing a non-contacting transducer. In an exemplary embodiment, the sensor mounts inside a hydraulic cylinder, away from the harsh environment, and provides a signal indicative of the position of the piston. The sensor provides a connector, attached between a cylinder piston and a converting element, for sensing the displacement of the piston. The converting element converts the cylinder displacement to a proportional displacement of a translating member. A precision transducer senses the displacement of the translating member and provides an electrical output signal proportional to the piston movement or to the piston""s position.
In one exemplary sensor according of the principles of the invention, a flexible connector such as a cable is attached to the movable element (a piston). The converting element comprises a pick-up spool coupled to the other end of the connector and rotatable about an axis. The spool is under tension from a recoil mechanism, such as a spring, coupled to the spool. A translating member, which can be a lead screw, engages threads on the interior of the spool, and translates along an axis when the spool rotates. A transducer is disposed to sense a position or motion of the translating member, and provides an output signal proportional to, and therefore indicative of, the position (or motion) of the translating member. The transducer can be a linear variable differential transformer (LVDT), which is a non-contacting transducer. Of course, other transducers, including those using contacting components can be used.
As a further feature of a sensor according to the principles of the invention, and as a still further exemplary embodiment thereof, there is provided a construction of the sensor frame by the use of a plurality of stamped plates that are contained within the hydraulic cylinder, preferably about five of such stamped plates and which stamped plates facilitate the ease and therefore reduce the cost of the constructing of an exemplary sensor, that is, with the use of a plurality of stamped plates, a frame for the sensor can be readily formed by the stamping process and which eliminates the need for specially complex machined blocks to thus reduce the cost of such construction. Also, with such embodiment, in addition to the considerable cost savings, there is a greater flexibility in the production of sensor frames of differing sizes by merely adapting the stamping techniques to produce the stamped plates of the appropriate dimensions for the particular desired size of sensor. As such, with relatively minimal tooling changes, the size of the various sensor frames can be changed, modified and adapted to accommodate a wide variety of dimensioned sensors to be located within the hydraulic cylinder.
As a still further exemplary embodiment, there is provided an improved mounting means whereby the sensor can be physically mounted within the hydraulic cylinder by utilizing the standard hydraulic threaded fluid ports that are normally found on such hydraulic cylinders. In this improved mounting means, use is made of the pair of standard hydraulic fluid ports that are located about 180 degrees apart on the periphery of the hydraulic cylinders. Flexible end caps comprised of a flexible material such as urethane, are positioned about the sensor and juxtaposed and in alignment with each of the fluid ports of the hydraulic cylinder. Two port inserts are then threaded, respectively into each of the standard fluid ports and those inserts are advanced by the user until they capture the sensor therebetween and thus sandwich the sensor comfortably but firmly between the port inserts to hold the sensor in a fixed position in place within the hydraulic cylinder. With the use of the flexible end caps, there is some inherent flexibility in the mounting means in order to isolate the sensor from shock and vibration that otherwise could affect the performance and long term durability of the sensor. There may also be some form of ribs, protrusions, button or any other molded feature that can enhance or add to the cushioning effect to provide the isolation of the sensor from the walls of the hydraulic cylinder. The port inserts are hollow such that the normal passage of the flow of hydraulic fluid is not impeded or occluded into and out from the hydraulic cylinder. In order to pass the electrical wires that are necessarily connected to the sensor located within the hydraulic cylinder to provide an outside connector to that sensor, i.e. for connection to external electrical equipment, such wiring is conveniently passed through one or both of the port inserts by a specially constructed high pressure seal assembly that maintains a sealed environment within the hydraulic cylinder and yet allows the wires to be connected to the equipment external of the cylinder.
In order to pass the electrical conductors through the wall of the hydraulic cylinder, there is a high pressure seal assembly that provides an electrical path for the sensor that is located within the high pressure environment of the cylinder to an external connector that is in the ambient environment so that some external electronic equipment can recognize the various signals from the sensor and interpret those signals to determine the position of the piston. The high pressure seal assembly therefore comprises a thermoplastic connector that cooperates with one of the aforedescribed hollow port inserts and which has a plurality of solid conductive pins that extend from a connector within the cylinder to an external connection in the outer environment. The pins are sealed within the plastic material of the connector and may be affixed therein by ultrasonic swaging or insert molding to insure a good seal along the solid conductive pins to prevent leakage from the high-pressure environment. The external peripheral surface of the connector can be sealed within the opening in the wall of the cylinder by means such as an O-ring. The eventual seal is relatively low cost and yet has the pressure resistance necessary for the application. As an advantage, the high pressure seal assembly according to the principles of the invention allows the use of the standard hydraulic fluid port already present in commercial hydraulic cylinders, and provides an inexpensive easily facilitated means of forming an electrical path from a high pressure environment to a environment normally at ambient atmospheric pressure.
As a still further feature, and which may be optional, there are provided piston stops within the hydraulic cylinder in order to protect the sensor. Since the sensor of this invention is preferably located within the hydraulic cylinder, it is possible during the normal operation of the hydraulic cylinder for the piston to be fully retracted and, in such case, the piston could encounter the sensor and crush that sensor. The piston stops are therefore incorporated as components of the construction of the sensor and its mounting means, such that the sensor can be safely located within the hydraulic cylinder at the back end thereof and which prevents the piston from contacting and potentially damaging the sensor. The piston stops can be constructed of a metal stamping and are formed to have an arcuate configuration to fit in a complementary relationship with the interior of the hydraulic cylinder. By the use of the piston stops, standard hydraulic cylinders can be used and the sensor is protected and wherein there is no need for the manufacturer of the hydraulic cylinders to build in costly stops or bumpers in the manufacturing of the cylinders themselves.
For use in a hydraulic cylinder, the sensor""s operation is like this. As the cylinder piston moves within the cylinder, the spool feeds out or draws in cable, thereby tracking the piston""s linear displacement. As the cylinder moves toward the spool, the spring causes the spool to wind the cable. When the cylinder moves away from the spool, the cylinder force overcomes the spring tension and pulls cable off the spool. The spool is in threaded engagement with a lead screw. As the spool rotates, the spool and lead screw converts the rotary motion of the spool to a linear displacement of the lead screw. The displacement is proportional to the piston displacement. The lead screw is attached to an LVDT core that moves within a LVDT body when the cylinder moves. The LVDT delivers an electrical signal at its output, which can be configured as a position signal, rate signal or the like.